Single drive system for controlling multiple independent gripper shafts

ABSTRACT

An independently driven symmetric cam individually opens and closes two different gripper shafts attached to an impression surface to grip and release leading edges of two different media sheets. Also disclosed is a single drive system for controlling multiple independent gripper shafts includes an impression cylinder, at least two gripper shafts on the impression surface of the impression cylinder to hold at least two media sheets to the impression surface, a dynamic cam to independently actuate each of the at least two gripper shafts, and a single drive motor to mechanically rotate the dynamic cam with respect to the impression surface.

The present application is a continuation-in-part, and claims thebenefit under 35 U.S.C. §120, of U.S. application Ser. No. 13/407,269,entitled “Cam Positioned to Retract a Mechanical Component” to Ofer BenBassat, et al., filed Feb. 28, 2012. This application is herebyincorporated by reference in its entirety.

BACKGROUND

Printers can use rotating drums to control print media and inkdeposition during a printing process. In some implementations, printmedia is secured to the outer surface of an impression cylinder duringdeposition of ink images. The impression cylinder rotates duringprinting to present the appropriate portion of the print media for thedeposition of the ink images. After the desired ink images are depositedon the print media, the print media is released from the impressioncylinder and the impression cylinder secures the next sheet of printmedia to its surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various examples of the principlesdescribed herein and are a part of the specification. The illustratedexamples are merely examples and do not limit the scope of the claims.

FIG. 1 is a diagram of a printer that includes an impression cylinderassembly, according to one example of principles described herein.

FIG. 2 is a side view of an impression cylinder assembly that includes asingle driving system for controlling multiple independent grippershafts, according to one example of principles described herein.

FIG. 3 is a cross sectional view of a dynamic cam and cam followermechanisms for a single driving system for controlling multipleindependent gripper shafts, according to one example of principlesdescribed herein.

FIG. 4 is a perspective view of an impression cylinder with a singledriving system for controlling multiple independent gripper shafts,according to one example of principles described herein.

FIG. 5 is a flowchart of a method for using an impression cylinder witha single driving system for managing print media during printing,according to one example of principles described herein.

FIG. 6 is a flow chart of a method for selectively closing and openinggripper sets to selectively grip and release print media, according toone example of principles described herein.

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements.

DETAILED DESCRIPTION

During the operation of a digital printing system, ink images are formedon the surface of a photo-imaging cylinder. These ink images aretransferred to a heated blanket cylinder and then to an impressioncylinder. The impression cylinder holds a print medium, such as a sheetof paper, on its surface to receive the ink images. The impressioncylinder receives print media from a feeding mechanism and secures theprint media throughout the ink deposition process. When the image on theprint medium is completed, the impression cylinder releases the printmedium and secures another sheet for printing. The press controlcylinder includes a number of mechanisms to control the position,timing, and sequential transfer of print media on the impressioncylinder.

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present systems and methods. It will be apparent,however, to one skilled in the art that the present apparatus, systemsand methods may be practiced without these specific details. Referencein the specification to “an example” or similar language means that aparticular feature, structure, or characteristic described in connectionwith the example is included in at least that one example, but notnecessarily in other examples.

FIG. 1 is a diagram of one illustrative example of a printing systemthat includes an impression cylinder. The desired image is initiallyformed on the photo-imaging cylinder (105). The desired image may betext, pictures, black/white images, partial color, full color images, orany combination of text and images. According to one example, thephoto-charging/laser imaging unit (110) charges portions of thephoto-imaging cylinder (105) that correspond to a first color of inkthat makes the desired image. A first binary ink developer (115)presents a uniform surface of ink to the photo imaging cylinder (105).The charged portions of the photo-imaging cylinder (105) attract the inkand form the desired ink pattern on the photo-imaging cylinder (105).This ink pattern is transferred to the blanket cylinder (120).

The sheet of print media enters the printing system (100) from theright, passes over the feed tray (125), and is wrapped onto theimpression cylinder assembly (130). The impression cylinder assembly(130) can secure the sheet to its surface in a number of ways, includingpulling the sheet against the surface using a vacuum, electrostaticadhesion, and mechanical clamping. In one example, the impressioncylinder of the impression cylinder assembly secures the leading edge ofthe sheet using grippers. The impression cylinder rotates on an axis topresent the appropriate portion of the media to the blanket cylinder(120). The blanket cylinder (120) transfers the ink pattern to the sheetas the sheet passes between the blanket cylinder (120) and theimpression cylinder (130). To form a single color image (such as a blackand white image), one rotation of the sheet with the impression cylindercompletes the desired image. For a multiple color image, the sheet isretained on the impression cylinder and makes multiple contacts with theblanket cylinder (120). At each contact, an additional color is placedon the sheet of print media. For example, to generate a four-colorimage, the photo-charging unit (110) forms a second pattern on thephoto-imaging cylinder (105) that receives the second ink color from asecond binary ink developer. As described above, this second ink patternis transferred to the blanket cylinder (120) and impressed onto thesheet as it continues to rotate with the impression cylinder. Thiscontinues until the desired image is formed on the sheet of print media.In other embodiments, multiple colors may be deposited simultaneously.

After the desired image is formed on a single sided print, theimpression cylinder assembly (130) passes the printed sheet to theperfector (135) that moves the sheet to the exit guide (145). Fordouble-sided prints, the perfector (135) and duplex conveyor (140)perform the more complex task of reversing the sheet and reintroducingthe sheet to the impression cylinder assembly so that the blank surfaceof the sheet is on the outside of the impression cylinder assembly (130)to receive the second image.

FIG. 2 is a side view of an impression cylinder assembly (130) thatincludes a single driving system for controlling two independent grippershafts (240). The impression cylinder assembly (130) includes animpression cylinder (210) that is supported by bearing housings (205);these bearing housings are mounted to? the press walls. The impressioncylinder (210) is operated by a main drive, which is not shown. The maindrive (210) may be used to synchronously drive a number of components,including the photo imaging cylinder (105, FIG. 1) and blanket cylinder(120, FIG. 1).

In this example, the impression cylinder assembly (130) is in place tosecure two media sheets at once to the impression cylinder (210).However, the principles discussed herein could be applied to animpression cylinder assembly that secures 2, 3, 4 or more media sheetsto the impression cylinder at one time. In this example, the impressioncylinder (210) includes two independent gripper shafts (240) thatmanipulate a set number of grippers (242). The gripper shafts (240)rotate a first direction to lift the grippers (242) above thecylindrical surface of impression cylinder (210) and rotate an oppositedirection to clamp the grippers (242) against the cylindrical surface ofthe impression cylinder (210). In FIG. 2, the grippers (242) grasp theleading edge of the media sheet (215) to secure it to the impressioncylinder (210).

The single drive system for controlling the two shafts includes a drivemotor (200), a motor gear (230), and a driven gear (215). The drivengear (215) is connected to a dynamic cam (235). The dynamic cam (235)and driven gear (215) are driven about a central axis by the drive motor(200). The angular position of the dynamic cam (235) and driven gear(215) is mechanically independent of the angular position of theimpression cylinder (210).

Two cam followers (220) are connected to the impression cylinder androll over the cam (235) and control the rotation of the gripper shafts(240) and the grippers (242) connected to the shafts. By selectivelyrotating the cam (235) with respect to the impression drum, the camfollowers (220) are lifted/dropped to close/open the grippers (242),thereby securing/releasing the leading edge of the media sheet (215).

FIG. 3 is a cross sectional view of a cam and cam follower mechanismsfor a single driving system for controlling two independent grippershafts taken along line A-A in FIG. 2. From a strictly mechanicalperspective, portions of the cam follower mechanisms (250), includingthe spring mechanisms (252) would be behind the cam (235) and obscuredfrom view. However, for purposes of illustration, all of the camfollower mechanisms have been shown in FIG. 3.

FIG. 3 shows more detail about the operation of the dynamic cam (235),cam follower mechanisms (250) and the rotation of the shafts (240) andgrippers (242). The two-gripper shafts (240) are located on oppositesides of the impression cylinder. As shown in FIG. 2, each shaft (240)extends along the length of the impression drum and actuates a set ofgrippers (242). One end of the shaft is clamped in the follower body(225). The follower body connects the shaft to the cam follower (220)and the spring mechanisms (245). The spring mechanisms (245) provide abiasing force that presses the cam followers (220) against the camsurface profile (237). This ensures that the cam followers (220) are incontact with the cam surface profile. In this example, the cam surfaceprofile (237) is primarily circular with an indented portion (238) thatallows the cam follower (220) to drop a predetermined distance towardthe center of the cam (235). In the example shown in FIG. 3, a first camfollower (220-1) is positioned over the indented portion (238). Thespring force generated by the spring mechanism (245-1) presses on thecam body (225-1) and forces the cam follower (220-1) the indentedportion (238). As the cam follower (220-1) drops into the intendedportion (238), the cam follower assembly (250-1) rotates and opens thegrippers (242-1) along the length of the gripper shaft (240-1). Thisreleases a media sheet from the surface of the impression drum (210) andallows another sheet to be inserted. The dynamic cam (235) is thenrotated with respect to the impression cylinder (210) to force the camfollower (220-1) upward and close the grippers to grasp the leading edgeof the media sheet.

The second cam mechanism (250-2) is shown with the cam follower (220-2)riding on the circular portion of the cam surface profile (237). The cam(220-2) is more radially distant from the center of the cam (235) androtates the shaft (240-2) so that grippers (242-2) closed. The grippers(242-2) in their closed position do not extend substantially above thesurface of the impression drum (210) and are mostly hidden behind aportion of the cam follower mechanism (250-2) in this view.

As can be seen in FIG. 3, in this implementation there is only oneindented portion (238) in the cam (235). Consequently, only one set ofgrippers are allowed to be opened at a time. FIG. 3 shows three separateactivation regions (305, 310, and 315). The loading region (305)represents an angular position of the impression drum (210) where thegrippers need to be open to receive a new sheet of media from the feedtray (125, FIG. 1). The duplex region (310) represents the region wherethe grippers need to be open to receive a sheet that has been reversedby the perfector (135) and duplex conveyor (140) so that it can beprinted on the opposite side. The exit region (315) represents theregion where grippers need to open to allow the perfector to remove asheet from the impression drum. The location and size of these regionsis only illustrative. The regions could have different sizes andlocations, subject to the constraints described below. Because theexample drive system can only open one of the gripper sets at a time,the actuation regions (305, 310, 315) should not be placed so that bothgripper sets need to be open at the same time. Consequently, in this twosheet system, if the regions are not located directly opposite eachother (180 degrees apart), there will not be a specific instant thatboth grippers need to be open at the same time. Similarly, the regionscan be located around the impression cylinder assembly to avoid the needto simultaneously open gripper sets in systems that have 3, 4, or moresheets on an impression cylinder at the same time. However, thisoperational constraint is not absolute. A system that does not meet thisoperational constraint can still be operated by opening one of thegripper sets to allow entry/exit of a leading edge of a media substratewhile the other sheet of media continues to be gripped for anotherrevolution of the impression drum. This reduces the throughput of thesystem, but allows the system to continue to operate.

FIG. 4 is a perspective view of an impression cylinder assembly (130)with a single driving system (200, 230, 215) for controlling multipleindependent gripper shafts (240). As discussed above, the impressioncylinder assembly (130) includes an impression surface (210) and grippershafts (240) on the impression cylinder (210) to hold at least two mediasheets to the impression surface. An independently driven symmetric cam(235) individually opens and closes the two different gripper shafts(240) attached to the impression cylinder (210) to grip and releaseleading edges of two different media sheets. The dynamic cam (235)independently actuates each of the gripper shafts (240). In someimplementations, the gripper shafts (240) are spaced 180 degrees aparton the impression surface. The single drive motor (200) mechanicallyrotates the dynamic cam (235) with respect to the impression surface(210). The gripper shafts (240) include a plurality of grippers (“agripper set”) that grips a leading edge of a media sheet placedunderneath them. A cam follower assembly (250) includes a cam follower(220) that travels over a profile surface of the dynamic cam (235) andis connected to the gripper shafts (240).

In some implementations, the profile surface of the dynamic cam (235) issymmetric and activates only one of the two-gripper shafts at a time. Inother embodiments, the cam may not be symmetric and different driveprofiles can be used for each rotational direction. Entry and exitregions of the media sheets onto/off of the impression cylinder (210)are arranged in such a way that the multiple media sheets do not enteror exit the impression cylinder (210) simultaneously. This allows thedynamic cam (235) to sequentially activate the gripper shafts (240) oneat a time to accomplish the desired media motion. The dynamic cam (235)rotates about to be on the same axis as the impression cylinder (210).In some examples, the rotational axis of the impression cylinder (210)is offset slightly to accommodate thicker print media, while the dynamiccam (235) continues to rotate around the bearing housing that is fixedto wall of the print housing. However, this slight offset of theimpression cylinder (210) does not adversely affect the operation of thedynamic cam (235) and activation of the grippers (242).

As discussed above, a main drive mechanism drives the impressioncylinder, blanket cylinder, and image cylinder at a near constantvelocity during printing. The single drive motor (200) can rotate thedynamic cam (235) at the same speed as the impression cylinder (210) ifno change in the position of the grippers (242) is desired or in aposigrade (faster than the impression cylinder) or retrograde (slowerthan the impression cylinder) relative to the impression cylinder (210)to change the position of the grippers (242).

FIG. 5 is a flowchart of a method for using an impression cylinder witha single driving system for managing print media during printing. FIG. 5shows a number of steps used to load two sheets of print media onto animpression drum, print the desired images onto the sheets and thenremove the sheet from the impression drum. In the first block, the camis rotated with respect to the impression cylinder so that a firstgripper shaft opens grippers to receive a media sheet from a feed tray(block 505). To do this, the drive motor (230) engages the driven gear(215) and rotates the attached dynamic cam (235) faster or slower thanthe impression cylinder (210). In this example, the dynamic cam (235) isrotated faster than the impression cylinder (210). The impressioncylinder (210) is typically driven at a constant speed by a main drive.The main drive also rotates the blanket cylinder and imaging cylinder insynchronization with the impression cylinder. By driving the dynamic camat speeds that are faster or slower than the impression cylinder, therotational position of the cam relative to the impression cylinderchanges. In the examples shown in FIG. 5, the cam is rotated faster thanthe impression drum to change the relative position of the dynamic camwith respect to the impression drum. This changes the position of thecam follower mechanism (250-1) to rotate the gripper shaft and to openthe gripper (242-1). In the series of drawings that accompany theflowchart, the gripper has been enlarged and positioned above theimpression cylinder surface so that its motion can be more clearlyillustrated.

The impression cylinder (210) continues to rotate. The dynamic cam isrotated at a differential speed so that the first gripper shaft closesthe set of grippers to grasp the leading edge of the media as it isloaded onto the impression cylinder (210) (block 510). In this example,the cam (235) is illustrated as rotating faster than the impressioncylinder (210). This moves the cam so that the cam follower is pushedradially outward and rotates cam assembly (250-1) and gripper shaft.This rotation of the gripper shaft closes the gripper (242-1) andpinches the leading edge of the first media sheet between the gripperand the impression cylinder (210).

As the impression cylinder (210) rotates at a constant speed, thedynamic cam continues to rotate faster than the impression cylinder sothat the second gripper shaft opens the second set of grippers toreceive a second media sheet (502-2). Simultaneously, the blanketcylinder (120) begins transferring a first ink image onto the firstmedia sheet as it passes through the pinch point between the impressioncylinder (210) and the blanket cylinder (120) (block 515). Due to thecam profile, actuating one of the gripper shafts can't affect/open thesecond gripper shaft.

The dynamic cam (235) continues to be driven at a differential speedwith respect to the impression cylinder (210) so that the second grippershaft (240-2) closes a second set of grippers (242-2) to grasp theleading edge of the second media sheet as it is loaded onto theimpression cylinder (210) (block 520). The two sheets (502-1, 502-2) arenow loaded onto the impression cylinder and the trailing portion of thefirst sheet is receiving the last of the first ink image from theblanket cylinder (120).

At this condition, the positions of both cam followers are known by thecontrol system. The control system is ready to rotate the cam to operatethe grippers as needed by the printing sequence. The dynamic cam (235)is rotated at the same velocity as the impression cylinder (210) to keepthe cam the same position relative to the impression cylinder. Thisensures that the first and second gripper shafts are closed and securesthe leading edges of the two media sheets (502) to the impressioncylinder (210). The impression cylinder and cam continue to rotate untilall the ink images are deposited on the first media sheet (block 525).For example, if a four-color image is being created on the first andsecond media sheets, the impression cylinder will rotate so that thefirst media sheet passes through pinch point between the blanketcylinder and the impression cylinder four times. Each time the firstmedia sheet passes through the pinch point, the blanket cylinderdeposits one of the four ink images on the media sheet. The retention ofeach sheet on the impression drum is independent from the other sheet.For example, a four color image could be printed on the first sheetwhile four one color images are sequentially printed.

When the first media sheet has received all the desired ink images, thecam is rotated at a differential speed with respect to the impressioncylinder so that the first gripper shaft opens to release the leadingedge of the first sheet (block 530). The perfector (135) or othermechanism removes the first sheet after the grippers (242-1) havereleased it. The second sheet continues to rotate with the impressioncylinder and passes through the pinch point to receive any additionalink images. The first media sheet (502-1) may be replaced by performingblocks 505 and 510. When printing on the second media sheet (502-2) iscomplete it can be removed in a similar manner to the first media sheet.In general, the retention a sheet on the impression drum is independentof any other sheet. Each sheet can be removed after it has been printed.

In some situations, the media sheets may be reversed and fed back ontothe impression cylinder for duplex printing. In other scenarios, onlyone side of the media is printed and the sheets are passed over an exitguide to a drying station, collator, or catch tray.

The motion of the cam with respect to the impression cylinder may becalculated in real time or may be predetermined for each scenario. Themotion of the cam may be determined to optimize a number of parameters,including minimizing energy consumed by the cam drive motor, the maximumspeed allowed by the motor performance, reducing acceleration anddeceleration values, reducing motor heating, reducing load on the gearsystem, or other parameters. The opening and closing of the grippers canoccur at fixed positions relative to the feed tray, blanket cylinder, orperfector. However, due to the dynamic nature of the mechanism this isnot a requirement. The location at which the grippers open and close canbe adjusted by altering the motion profile of the dynamic cam.

FIG. 6 is a flow chart of a method for selectively closing and openinggripper sets to selectively grip and release print media. A dynamic camis rotated relative to an impression cylinder using a single drive toopen a first gripper shaft on the impression cylinder (block 605).Rotating the dynamic cam relative to the impression cylinder can beperformed by speeding or slowing the rotation of the cam with respect tothe impression cylinder such that the cam is positioned to release orgrip the first media sheet at a predetermined time and angle.

The dynamic cam is rotated relative to the impression cylinder to closethe first gripper set after a leading edge of a first media sheet isinserted under the first gripper set (block 610). The dynamic cam isrotated relative to the impression cylinder to open a second gripper seton the impression cylinder (block 615). The dynamic cam is rotatedrelative to the impression cylinder to close the second gripper setafter a leading edge of a second media sheet is inserted under thesecond gripper set (block 620). The dynamic cam is rotated at the samerotational velocity as the impression cylinder to maintain the first andsecond gripper set in closed positions during transfer of ink images tothe first and second media sheets until printing of one of the first andsecond media sheets is complete (block 625). The method may also includerotating the dynamic cam relative to the impression cylinder to releaseone of the first and second media sheets when the printing of that sheetis complete. The motion profile of the cam can be generated to move thecam relative to the impression cylinder to selectively actuate one of aplurality of gripper sets on the impression cylinder. The cam motionprofile may be generated to accommodate a print mode to sequentiallyopen and close the gripper sets one at a time, and to minimize energyconsumed in moving the dynamic cam.

There are several advantages to using an impression cylinder that holdsmultiple sheets of media. Because their impression cylinder is bigger,its angular rotation may be slower and the curvature of the individualsheets may be less. This may allow for a wider variety of media to beprinted. For example, stiffer media may benefit from the lower amount ofcurvature of each sheet. Additionally, the slower angular velocities andbigger cylinder diameters provide each sheet with a longer drying timebetween successive depositions of ink images. This can produce higherquality images and allow for more flexibility in formulating inks.Another advantage is that the printed sheet may be released from thedrum after its trailing edge is fully printed and has passed under theblanket cylinder. This can significantly reduce registration errors.

The principles described above provide for a single drive thatindependently actuates multiple gripper sets. These multiple grippersets allow multiple media sheets to be attached to the impressioncylinder. The drive system controls the cam via a gear transmission. Thedrive system dynamically controls the cam using programmed motionprofiles. The motion profiles may be programmed according to theprinting mode and other press control sequences. Activation angles ofthe gripper shafts may be located at various locations to load andunload media sheets. In some implementations, the activation regions arespaced so that multiple gripper sets do not need to be simultaneouslyactivated. In order to allow activation of the gripper shafts in bothdirections, the cam may have a symmetric profile. This also simplifiesthe control formula. In some implementations, using a calibrationprocedure to precisely locate the cam's axis of symmetry relative to adrive motor encoder may accurately control the gripper shaft motion. Tosense the cam location with respect to the main encoder of the press, aproximity sensor may be used to sense the rotating cam. For example, theproximity sensor may be a mechanical or magnetic “home” trigger. Anotheroption is to use drive motor current tracking to control the actual camposition.

By using a single drive to manipulate multiple gripper sets, the costand complexity of the impression cylinder can be reduced withoutinterfering with the operation of the press. Further, the overall sizeof the impression cylinder assembly can be reduced because there is onlyone drive system instead of an individual drive system for each gripperset.

The preceding description has been presented only to illustrate anddescribe examples of the principles described. This description is notintended to be exhaustive or to limit these principles to any preciseform disclosed. Many modifications and variations are possible in lightof the above teaching.

What is claimed is:
 1. A single drive system for controlling multipleindependent gripper shafts comprising: an impression cylinder; at leasttwo gripper shafts on an impression surface of the impression cylinderto hold at least two media sheets to the impression surface; a dynamiccam to independently actuate each of the at least two gripper shafts;and a single drive motor to mechanically rotate the dynamic cam withrespect to the impression cylinder.
 2. The system of claim 1, in whicheach of the gripper shafts comprises a plurality of grippers to grip theleading edge of a media sheet.
 3. The system of claim 1, in which thecam follower travels over a profile surface of the dynamic cam and isconnected to the gripper shafts.
 4. The system of claim 3, in which theprofile surface of the dynamic cam is symmetric.
 5. The system of claim1, in which the dynamic cam activates only one of the at least twogripper shafts at a time.
 6. The system of claim 1, in which entryregions of media sheets onto the impression surface and an exit regionof the media sheets off of the impression surface are arranged such thatmultiple media sheets do not enter or exit impression surfacesimultaneously.
 7. The system of claim 1, in which dynamic cam rotatesabout substantially the same axis as the impression cylinder.
 8. Thesystem of claim 1, further comprising a main drive to drive theimpression surface, a blanket cylinder, and an image cylinder at aconstant velocity during printing, in which the imaging cylinder is togenerate ink images and the blanket cylinder is to transfer ink imagesfrom the imaging cylinder to the media sheets held on the impressionsurface.
 9. The system of claim 1, in which the single drive motor is torotate the dynamic cam in retrograde with respect to the impressionsurface to actuate a gripper shaft.
 10. The system of claim 1, in whichthe at least two gripper shafts comprise a first gripper shaft and asecond gripper shaft spaced 180 degrees apart on the impression surface.11. An independently driven symmetric cam to individually open and closetwo different gripper shafts attached to an impression surface to gripand release leading edges of two different media sheets.
 12. A methodfor printing comprising: rotating a dynamic cam relative to animpression cylinder using a single drive to open a first gripper set onthe impression cylinder; rotating the dynamic cam relative to theimpression cylinder to close the first gripper set after a leading edgeof a first media sheet is inserted under the first gripper set; rotatingthe dynamic cam relative to the impression cylinder to open a secondgripper set on the impression cylinder; rotating the dynamic camrelative to the impression cylinder to close the second gripper setafter a leading edge of a second media sheet is inserted under thesecond gripper set; rotating the dynamic cam at a same rotationalvelocity as the impression cylinder to maintain the first and secondgripper set in closed positions during transfer of ink images to thefirst and second media sheets until printing of one of the first andsecond media sheets is complete.
 13. The method of claim 12, furthercomprising rotating the dynamic cam relative to the impression cylinderto release one of the first and second media sheets when printing ofthat sheet is complete.
 14. The method of claim 12, further comprisinggenerating a cam motion profile to move the cam relative to theimpression cylinder to selectively actuate a plurality of gripper setson the impression cylinder, in which the cam motion profile is generatedto accommodate a print mode, to sequentially open and close each gripperset one at a time, and to minimize energy consumed in moving the dynamiccam.
 15. The method of claim 12, in which rotating the dynamic camrelative to the impression cylinder comprises speeding or slowing therotation of the dynamic cam with respect to the impression cylinder suchthat the dynamic cam is positioned to release or grip the first mediasheet at a predetermined time and angle.